The present invention concerns a method of diagnosing a start/stop unit of a motor vehicle motor. It relates more particularly to determining if said start/stop unit is short-circuited to ground.
In known manner, start/stop units (usually termed start/stop buttons) are used in motor vehicles to transmit to a computer controlling the motor an instruction to start or stop said motor.
Such units are notably employed in vehicles equipped with a hands-free access system that does not use a mechanical key. They are routinely fitted to vehicles incorporating a function for stopping the motor when the vehicle is immobile and automatically restarting as soon as the vehicle begins to move again (for example after the accelerator pedal or the clutch pedal is depressed). These so-called stop and go functions are well known in themselves and will not be described in detail here.
Start/stop systems 100 (see FIG. 1) consist of a start/stop device 10 controlled by a driver device 11. The start/stop device 10 conventionally includes three inputs outputs. Namely:                a power supply input 1,        a control output 2 delivering a control signal for starting and stopping a motor (not shown), and        a ground output 3.        
Said start/stop device 10 also includes a two-position start/stop unit (switch 4) connected by a conductive track 5 to the control output 2. The power supply input 1 of the start/stop device 10 also includes in known manner a light-emitting diode (LED 14) in series with a power supply resistor RA. This LED 14 has the function of lighting up to render the start/stop unit 4 visible.
FIG. 1 also shows a conventional circuit diagram of a driver device 11 including a computer 12, a control circuit 13 connected to the power supply input 1 of the start/stop device 10, a circuit 15 for reading the status of the start/stop unit and a ground return 16.
When a driver wishes to activate the start/stop unit 4, it suffices for them to depress the switch 4 to switch it to its closed position. When this is done, the input of the reading circuit 15 detects a signal value substantially equal to zero (or very low).
Now, the value read by the reading circuit 15 is also substantially equal to zero (or very low) if a short-circuit ground (shown by two broken lines CC) occurs on the conductive track 5 between the switch 4 and the output 2.
Two distinct states, namely a start/stop instruction and a short-circuit on the conductive track 5, have the same consequence in that the same value (zero) is detected by the reading circuit 15. It is therefore not possible to distinguish these two states from each other, which constitutes a serious risk of misinterpretation of the instruction given by the driver.
In fact, if a short-circuit to ground occurs on the conductive track 5, it may generate an (unwanted) instruction to start the motor, which constitutes a risk of malfunction critical for the use of the vehicle.
This risk is generally circumvented by the fact that other parameters exist that make it possible to verify that an instruction to start the motor has been given. For example, the control system of the vehicle will verify that the brake or the clutch pedal has also been activated. It has nevertheless appeared important to eliminate this risk of starting the motor with a short-circuit to ground present on the conductive track 5 in order to make the vehicle safer without calling for specific maneuvers to be performed by the driver (depressing the brake or the clutch pedal).